Method of and system for calibrating gas flow dilutors

ABSTRACT

The preferred embodiments described herein make possible to use lower cost, fixed flow components, such as critical orifices, which are fast to stabilise to a steady flow. These cannot be adjusted to achieve equal flows but are selected to be sensibly close to their desired flow values. This embodiment determines the true ratios of the flows of all of the flow controlling components. Actual flows are not measured but near equal flows are each fed to a common flow meter and the ratio of the indicated flow meter readings is taken to be the same as the ratio of the flows. Two, near equal flows are then combined and compared to a single flow of approximately the same value, and so on. The flow meter is used only to compare near equal flows so does not need to be calibrated nor linear over a wide range.

The present invention relates to a method and system for calibrating gas flow dilutors.

Standard gas mixtures are available in gas cylinders containing a fixed concentration of an analyte gas (for instance carbon monoxide) in a matrix gas (for instance nitrogen). Gas flow dilutors combine a flow of standard gas mixture with an additional flow of the relevant matrix gas (referred to as diluent gas) to provide a dilution. Adjustment of the ratio of the two flow rates provides a means of adjusting the concentration of analyte in the combined flow.

The present invention seeks to provide improved calibration of gas dilutors of a type which functions by turning “on” or “off”, in various combinations, a number of fixed flows of standard gas mixture and diluent gas. Such gas dilutor types are well known and often use capillaries, orifices or critical orifices as the fixed flow controlling components.

Gas dilutors are generally calibrated by measuring the actual flows of the flow controlling components with respect to external reference standards of flow. Such calibrations of commercially available gas dilutors are often performed annually, in calibration laboratories. This is expensive, time consuming and the accuracy may not be maintained throughout this period.

However, calibration of actual flows is unnecessary since dilution is a function of flow ratios and knowledge of actual flow values is not required. British patent GB-B-2,333,614 describes a method of calibrating a number of flow controlling devices to accurately set their flow ratios by adjusting a series of flow combinations to give equal readings on a common flow meter. A detailed description of a “high-accuracy gas flow dilutor using mass flow controllers with binary weighted flows”, which utilises this method, was published in Measurement Science and Technology 13 (2002) 1138-1145. The method may be used to perform calibrations practically at frequent intervals at the point of use.

The method of British patent GB-B-2,333,614 is only applicable to dilutors which use flow-controlling components that are adjustable and can be set to a desired flow, such as mass flow controllers. Mass flow controllers are large, expensive and slow to stabilise to an adjustment of their flow. An accurate calibration, which depends upon adjusting flows to be equal, can therefore be protracted.

Document U.S. Pat. No. 3,886,971 discloses a dilutor having fixed fluid flow components.

The present invention seeks to provide an improved method and system for calibrating gas flow dilutors.

According to an aspect of the present invention, there is provided a method of calibrating a gas flow dilutor including the steps of providing a plurality of fixed flow fluid components, these being selected to provide desired flow rates; obtaining a measure of fluid flow through the plurality of fixed flow components by feeding the flow from one or more of said components through a common flow meter and measuring the flows through the flow components and determining therefrom the ratios of the flows, thereby to obtain said measure of flow through the components.

According to another aspect of the present invention, there is provided apparatus for calibrating a gas flow dilutor including plurality of fixed flow fluid components operable to provide desired flow rates; a common flow meter coupled to receive fluid flow from said fluid flow components and operable to obtaining a measure of fluid flow through the plurality of fixed flow components, means for determining therefrom the ratios of the flows, thereby to obtain a measure of flow through the components.

The preferred embodiments described herein make possible to use lower cost, fixed flow components, such as critical orifices, which are fast to stabilise to a steady flow. These cannot be adjusted to achieve equal flows but are selected to be sensibly close to their desired flow values. This embodiment determines the true ratios of the flows of all of the flow controlling components. Actual flows are not measured but near equal flows are each fed to a common flow meter and the ratio of the indicated flow meter readings is taken to be the same as the radio of the flows. Two, near equal flows are then combined and compared to a single flow of approximately the same value, and so on. The flow meter is used only to compare near equal flows so does not need to be calibrated nor linear over a wide range.

An embodiment of the present invention is described below, by way of example only, with reference to the drawing in which FIG. 1 shows in schematic form an embodiment of flow control device and in particular for calibrating a gas flow dilutor.

Referring to FIG. 1, this shows, by way of example, an embodiment of a gas flow dilutor using critical orifices as the flow controlling components. The generic design is known but additional components, 7A, 8A and 11 are incorporated to make possible an improved method of calibration. It is preferred that component 6 is also included to improve the accuracy of the calibration.

Standard gas is supplied at input 1 to pressure regulator 4 via three-way tap 3 and feeds taps 7A to 7F. Diluent gas is supplied at input 2 to pressure regulator 5 and feeds taps 7A to 7F. These taps allow either standard gas or diluent gas to flow to critical orifices 8A to 8F. Additionally, each tap may be positioned to simultaneously shut off the flow of both gases.

The combined flows from the critical orifices flow to tap 10 and can be directed to the output port 9 or through flow meter 11 to exhaust port 12.

Tap 3 allows diluent gas to be conveniently fed to regulator 4 in place of standard gas in order to conserve standard gas during calibration.

Different dilutions of the standard gas are obtained by changing the combinations of settings of taps 7A to 7F.

Critical orifices 8A to 8F are selected to give nominal (for instance, within ±3% of value) flows of 1, 1, 2, 4, 8 and 16 units of flow respectively. (A unit of flow is chosen to provide the desired flow rate from the output.)

The output pressures from the regulators 4 and 5 are preferably equal and sufficient to ensure correct operation of the critical orifices.

Differential pressure meter 6 allows the output pressures of the two regulators 4 and 5 to be conveniently set to be equal. It also allows changes in the output pressures, which may result from changes in flow, to be monitored and compensated for.

The flow meter 11 does not need to be accurately calibrated, nor highly linear, as it is used only to compare pairs of flows that are close to being equal.

The preferred method of calibration is as follows:

-   -   1) diluent gas is connected to port 2 and its pressure adjusted         to be sufficient to supply the regulators 4 and 5;     -   2) tap 3 is set to supply diluent gas to both regulators;     -   3) taps 7B to 7F are set to off (no flow);     -   4) tap 7A is set to supply diluent gas to critical orifice 8A;     -   5) tap 10 is set to direct the flow through the flow meter 11;     -   6) the flow from the diluent regulator 5 through orifice 8A will         be referred to as FD(8A) and is conveniently used as the         reference flow to which all other flows are compared. The         corresponding indication of flow on meter 11 will be referred to         as ID(8A);     -   7) tap 7A is turned to off and tap 7B set to provide flow from         regulator 5 to orifice 8B. The flow FD(8B) gives indication         ID(8B);     -   8) then, to a close approximation:

FD(8B)=FD(8A). ID(8B)/ID(8A);

-   -   9) both taps 7A and 7B are set to flow gas from regulator 5.         Using a similar nomenclature, the combined flow FD(8A+8B) is         indicated on meter 11 as ID(8A+8B);     -   10) taps 7A and 7B are set to off. Tap 7C is set to flow from         regulator 5 to achieve flow FD(8C) and indication ID(8C);     -   11) then, to a close approximation:

FD(8C)=FD(8A+8B). ID(8C)/ID(8A+8B);

-   -   12) by a similar process of combining flows and comparing with a         similar flow the following close approximations may be         determined:

FD(8D)=FD(8A+8B+8C). ID(8D)/ID(8A+8B+8C)

FD(8E)=FD(8A+8B+8C+8D). ID(8E)/ID(8A+8B+8C+8D)

FD(8F)=FD(8A+8B+8C+8D+8E). ID(8F)/ID(8A+8B+8C+8D+8E).

This provides sufficient information to determine the relative flow of critical orifices 8B to 8F when supplied from regulator 5, and with respect to the flow through orifice 8A, also supplied from regulator 5;

-   -   13) steps 4) to 12) are then repeated, but the taps are set to         supply gas from pressure regulator 4 rather than regulator 5.         These flows and indications will be referred to as FS and IS,         respectively and the following close approximations may be         determined:

FS(8B)=FS(8A). IS(8B)/IS(8A)

FS(8C)=FS(8A+8B). IS(8C)/IS(8A+8B)

FS(8D)=FS(8A+8B+8C). IS(8D)/IS(8A+8B+8C)

FS(8E)=FS(8A+8B+8C+8D). IS(8E)/IS(8A+8B+8C+8D)

FS(8F)=FS(8A+8B+8C+8D+8E). IS(8F)/IS(8A+8B+8C+8D+8E)

This provides sufficient information to determine the relative flows of critical orifices 8B to 8F when supplied from regulator 4, and with respect to the flow through orifice 8A, also supplied from regulator 4.

-   -   14) Using the flow meter 11, the flow through orifice 8A when         supplied from regulator 4 is compared to the flow obtained when         supplied from regulator 5. Thus:

FS(8A)=FD(8A). IS(8A)/ID(8A)

-   -   15) The relative flows of diluent gas or standard gas through         each of the orifices can then be calculated.

In operation, taps 7A to 7F are used in combination to set up flows of standard gas and diluent gas. The relative flow of the two gases defines the dilution.

In practice, the output pressure of the two regulators 4 and 5 may reduce with increasing flow through them. This would result in a reduction of flow through the orifices and a consequent error in the determined relative flows and hence, the dilution. This effect may be small enough to be ignored but pressure meter 6 can be used to measure the relationship between pressure and flow and to apply compensation.

There are various modifications possible to the above described embodiment. For instance, flow controlling devices such as capillaries or non-critical orifices could be used. The number of flow controlling devices may be increased or decreased. The nominal flows of the flow controlling devices need not follow a binary weighted relationship (for example 1,1,1,3,5 and 10 units of flow). The flow meter could be selected from two or more in order to increase the range of measurement. The regulated pressures could be individually monitored. The order of making the calibration measurements could be varied. The processing of the calibration data could be varied. The system could be used to provide linear flow control of a single gas.

The calibration method and apparatus disclosed herein can be incorporated into existing types of gas flow dilutors so that they can be frequently (for instance, daily) recalibrated by the user to remove the effects of long-term drift. This produces a potentially more accurate dilutor (0.1% relative uncertainty has been achieved) and removes the cost and downtime associated with sending the dilutor to a laboratory for regular recalibration. 

1-19. (canceled)
 20. A method for the calibration of a flow dilutor comprising a diluent gas port (2) and a standard gas port (1), a diluent pressure regulator (4) and a standard pressure regulator (5) both connected by means of a tap (7A) to a calibrating fixed flow component (8A) and by means of taps (7B, . . . 7F) to each of a plurality of fixed flow components (8B . . . 8F), a first (8B) of said plurality of fixed flow components having substantially the same flow rate as the calibrating fixed flow component (8A), the outlet of all fixed flow component (8A . . . 8F) being interconnected and forming an outlet of the gas flow dilutor; said method being characterised by the steps of: 1—delivering diluent gas through the diluent pressure regulator (5) to the calibrating fluid flow component (8A) only, 2—connecting the common output of the dilutor to a flow meter (11) and reading the indication corresponding to said flow FD8A, 3—using the flow of diluent gas through the calibrating fluid flow component (8A) as a reference flow IDEA to which all other flows will be compared, 4—delivering diluent gas through the diluent pressure regulator only to the first (8B) fluid flow component of said plurality of fluid flow components and reading the indication ID8B of the flow meter (11) for the flow FD8B through said fluid flow component (8B), 5—making the approximation FD8B=FD8A.ID8B/ID8A, 6—delivering diluent gas through the diluent pressure regulator to the calibrating fluid flow component (8A) and to the first (8B) of said plurality of fluid flow components (8B . . . 8F) and reading the indication ID(8A+8B) on the flow meter (11) corresponding to said combined flow FD(8A+8B), 7—delivering diluent gas through the diluent pressure regulator to the second (8C) of said plurality of fluid flow components (8B . . . 8F) only and reading the corresponding indication ID(8C) on the flow meter (11), 8—making the approximation FD(8C)=FD(8A+8B).ID8C/ID(8A+8B), 9—by a similar process of combining and comparing with a similar flow the following close approximations may be determined: FD(8D)=FD(8A+8B+8C),ID(8D)/ID(8A+8B+8C) FD(8E)=FD(8A+8B+8C+8D),ID(8E)/ID(8A+8B+8C+8D) FD(8F)=FD(8A+8B+8C+8D+8E),ID(8F)/ID(8A+8B+8C+8D+8E) determining from these approximations the relative flow of fluid flow components 8B to 8F when supplied with diluent gas through the diluent pressure regulator and with respect to the flow through the calibrating fluid flow component (8A), also supplied with diluent gas, through the diluent pressure regulator, 10—steps 1 to 9 are then repeated, but the fluid flow components (8A . . . 8F) are fed with diluent gas through the standard pressure regulator (4), these flows and indications will be referred to as FS and IS, respectively and the following approximations may be determined: FS(8B)=FS(8A).IS(8B)/IS(8A) FS(8C)=FS(8A+8B).IS(8C)/IS(8A+8B) FS(8D)=FS(8A+8B+8C).IS(8D)/IS(8A+8B+8C) FS(8E)=FS(8A+8B+8C+8D).IS(8E)/IS(8A+8B+8C+8D) FS(8F)=FS(8A+8B+8C+8D+8E).IS(8F)/IS(8A+8B+8C+8D+8E) determining from these approximations, the relative flows of fluid flow components (8B to 8F) when supplied with diluent gas through the standard pressure regulator and with respect to the calibrating fluid flow component (8A) also supplied with diluent gas through the standard pressure regulator, 1—using the flow meter (11), the flow through the calibrating fluid flow component (8A) when supplied with standard gas from standard pressure regulator is compared to the flow obtained when supplied with diluent gas through standard pressure regulator; thus: FS(8A)=FD(8A).IS(8A)/ID(8A), 12—the relative flows of diluent gas or standard gas through each of the fluid flow components can then be calculated.
 21. A method according to claim 20, wherein at least one of said components is coupled to provide diluent gas in said dilutor and at least one other of said components is coupled to provide standard gas to be diluted.
 22. A method according to claim 20, wherein said fixed flow fluid components are provided with critical orifices, capillaries or non-critical orifices.
 23. A method according to claim 20, wherein said fixed flow components have capacities of at least 1, 1 and 2 units of flow respectively.
 24. A method according to claim 20, including the step of feeding standard fluid and diluent fluid into the dilutor through the diluent pressure regulation and the standard pressure regulator, wherein said flow regulators are equal in capacity.
 25. A method according to claim 24, including the step of measuring differential pressure between said regulators and compensating for pressure differences therebetween.
 26. A method according to claim 20, including the step of calibrating the other fluid flow components on the basis of relative flow of fluid through said other components and combinations of one or more of the other fluid components of the dilutor.
 27. A method according to claim 20, wherein calibrated fluid flow through one or more of said fluid flow components is used to calibrate one or more flow regulators of the dilutor.
 28. A method according to claim 20, including the step of providing frequent calibration to compensate for long term drift by means of said flow meter.
 29. Apparatus to carry out the method of claim 20, including a gas flow dilutor comprising a diluent gas port (2) connected to a diluent pressure regulator (5) and a standard gas port (1) connected to a standard pressure regulator (4), both said regulator being connected by means of a tap (7A) to a calibrating fixed flow component (8A) and by means of taps (7B, . . . 7F) to each of a plurality of fixed flow components (8B . . . 8F), the first one (8B) having substantially the same flow rate as the calibrating fixed flow component (8A), the outlet of all fixed flow component (8A . . . 8F) being interconnected and forming an outlet of the gas flow dilutor; characterised by the fact that said apparatus comprises further a three way valve (19) fed by the common outlet of the dilutor, one of the outlets of said valve (10) directing the output flow of the common outlet of the dilutor to a common flow meter (11) receiving fluid flow from said fixed fluid component (8B . . . 8F) and giving a measure of fluid flow through the plurality of fixed flow components (8B . . . 8F) and means for determining therefrom the ratios of the flows.
 30. Apparatus according to claim 29, wherein said fixed flow fluid components are provided with critical orifices, capillaries or non-critical orifices.
 31. Apparatus according to claim 29, wherein said fixed flow components have capacities of at least 1, 1 and 2 units of flow respectively.
 32. Apparatus according to claim 29, including first and second pressure regulators for feeding standard fluid and diluent fluid through the dilutor, wherein said pressure regulators are equal in capacity. 